1. Field of the Invention
The present invention is related to marine geophysical exploration. More specifically, the invention is related to sensors for detecting seismic signals and to marine seismic data gathering.
2. Description of Relevant Art
In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth at the surface and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic wave is reflected when there is a difference in impedance between the layer above the interface and the layer below the interface.
In marine seismic exploration, a seismic shock generator, such as an airgun, for example, is commonly used to generate an acoustic pulse. The resulting seismic wave is reflected back from subsurface interfaces and detected by sensors deployed in the water or on the water bottom.
In a typical marine seismic operation, a streamer cable is towed behind an exploration vessel at a water depth between about six to about nine meters. Hydrophones are included in the streamer cable for detecting seismic signals. A hydrophone is a submersible pressure gradient sensor that converts pressure waves into electrical signals that are typically recorded for signal processing, and evaluated to estimate characteristics of the earth's subsurface.
After the reflected wave reaches the streamer cable, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the hydrophones in the streamer cable. The reflection coefficient at the surface is nearly unity in magnitude and negative in sign. The seismic wave will be phase-shifted 180 degrees. The downwardly traveling wave is commonly referred to as the “ghost” signal, and the presence of this ghost reflection creates a spectral notch in the detected signal. Because of the spectral notch, some frequencies in the detected signal are amplified and some frequencies are attenuated.
Because of the ghost reflection, the water surface acts like a filter, making it difficult to record data outside a selected bandwidth without excessive attenuation or notches in the recorded data.
Maximum attenuation will occur at frequencies for which the distance between the detecting hydrophone and the water surface is equal to one-half wavelength. Maximum amplification will occur at frequencies for which the distance between the detecting hydrophone and the water surface is one-quarter wavelength. The wavelength of the acoustic wave is equal to the velocity divided by the frequency, and the velocity of an acoustic wave in water is about 1500 meters per second. Accordingly the location in the frequency spectrum of the resulting spectral notch is readily determinable. For example, for a streamer water depth of 7 meters, as illustrated by curve 54 in FIG. 1, maximum attenuation will occur at a frequency of about 107 Hz. and maximum amplification will occur at a frequency of about 54 Hz.
It has not been practical to tow cables deeper than about 9 meters because the location of the spectral notch in the frequency spectrum of the signal detected by a hydrophone substantially diminishes the utility of the recorded data. It has also not been practical to tow cables at a depth shallower than about 6 meters, because the ghost signal reflected from the water surface substantially attenuates the signal detected by a hydrophone within the frequency band of interest.
It is also common to perform marine seismic operations in which sensors are deployed on the water bottom. Such operations are typically referred to as “ocean bottom seismic” operations. In ocean bottom seismic operations, both hydrophones and geophones are employed for recording the seismic data, with the geophone normally being placed in direct contact with the ocean bottom. To improve the contact between the geophone and the ocean floor, the geophone assembly is typically made to be quite heavy, with a typical density of between 3 and 7 grams per cubic centimeter.
A geophone detects a particle velocity signal, whereas the hydrophone detects a pressure gradient signal. The geophone has directional sensitivity, whereas the hydrophone does not. Accordingly, the upgoing wavefield signals detected by the geophone and the hydrophone will be in phase, but the downgoing wavefield signals detected by the geophone and the hydrophone will be 180 degrees out of phase. Various techniques have been proposed for using this phase difference to reduce the spectral notch caused by the ghost reflection.
U.S. Pat. No. 4,486,865 to Ruehle, for example, teaches a system said to suppress ghost reflections by combining the outputs of pressure and velocity detectors. The detectors are paired, one pressure detector and one velocity detector in each pair. A filter is said to change the frequency content of at least one of the detectors so that the ghost reflections cancel when the outputs are combined.
U.S. Pat. No. 5,621,700 to Moldovenu also teaches using at least one sensor pair comprising a pressure sensor and a velocity sensor in an ocean bottom cable in a method for attenuating ghosts and water layer reverberations.
U.S. Pat. No. 4,935,903 to Sanders et al. teaches a marine seismic reflection prospecting system that detects seismic waves traveling in water by pressure sensor-particle velocity sensor pairs (e.g., hydrophone-geophone pairs) or alternatively vertically-spaced pressure sensors. Instead of filtering to eliminate ghost reflection data, the system calls for enhancing primary reflection data for use in pre-stack processing by adding the ghost data.
U.S. Pat. No. 4,979,150 provides a method for marine seismic prospecting said to attenuate coherent noise resulting from water column reverberation by applying a scale factor to the output of a pressure transducer and a particle velocity transducer positioned substantially adjacent one another in the water. In this method, the transducers may be positioned either on the ocean bottom or at a location in the water above the bottom, although the ocean bottom is said to be preferred.
Four component system have also been utilized on the sea floor. A four component system utilizes a hydrophone for detecting a pressure signal, together with a three-component geophone for detecting particle velocity signals in three orthogonal directions: vertical, in-line and cross line. The vertical geophone output signal is used in conjunction with the hydrophone signal to compensate for the surface reflection. The three orthogonally positioned geophones are used for detecting shear waves, including the propagation direction of the shear waves.
The utility of simultaneously recording pressure and vertical particle motion in marine seismic operations has long been recognized. However, a geophone (or accelerometer) for measuring vertical particle motion must be maintained in a proper orientation in order to accurately detect the signal. Maintaining such orientation is non-trivial in a marine streamer and significantly more problematic than maintaining such orientation on the ocean bottom. Exploration streamers towed behind marine vessels are typically over one mile in length. Modern marine seismic streamers may use more than 10,000 transducers. To maintain a particle velocity sensor (a geophone or accelerometer) in proper orientation to detect vertical motion, the prior art has proposed various solutions. The use of gimbals has been proposed repeatedly. One example is a “gimbal lock system for seismic sensors” described in U.S. Pat. No. 6,061,302 to Brink et al. Another example is a “dual gimbal geophone” described in U.S. Pat. No. 5,475,652 to McNeel et al. Still another example is a “self-orienting directionally sensitive geophone” described in U.S. Pat. No. 4,618,949 to Lister. Nevertheless, no streamers containing both hydrophone and geophones are in commercial use.
In addition to the problem of maintaining orientation, severe noise from streamer cables has been considered prohibitive to use of particle velocity sensors in streamers. Because the voltage output signal from particle velocity sensors is normally not as strong as the output signal from hydrophones, the noise level in streamer cables has been a detriment to the use of particle velocity sensors.
In ocean bottom cables, the sensors are located on the sea floor and therefore are less exposed to noise generated by vibrations in the cable. Geophones are typically gimbaled to ensure a correct direction and are made of heavy brass or similar material to ensure good contact with the sea floor. The geophone housing is typically filled with fluid to improve the coupling between the sensor and the seafloor. However, because of the variation in properties of the seafloor from location to location, impedance mismatch between the seafloor and the sensor and sensor housing can cause problems. Such mismatch in impedance can cause various types of distortion in both the hydrophone signal and the geophone signal. Also, the boundary effects for the hydrophone and the geophone due to their closeness to the sea floor can change the response for the hydrophone and the geophone, giving rise to a need to correct the amplitude values in processing to be able to use the signal for elimination of the surface “ghost” reflection.
Accordingly a need continues to exist for an improved system for gathering marine seismic data.